Process for blackening copper or copper alloy surfaces



, one to eight hours.

.weeks.

Patented Dec. 12, a

PROCESS FOR BLACKENING COPPER OR COPPER ALLOY SURFACES Walter R. Meyeiyflamden, Conn.

' No Drawing. Application December 29, 1942, Serial No. 470,510

, 3 Claims.

The present invention relates to a process and composition for blackening'copper and copper alloys.

Heretoiore, copper and copper alloy surfaces have been blackened by the so-called oxidizing process in which the surface is cleaned and a copper sulphide film is formed by immersing the surface in solutions of sodium sulphide or ammonium sulphide or other water soluble sulphides. This produced brown or black coatings on the sur-' face. These sulphide films are not stable to oxidation, particularly when exposed to outdoor atmospheres, and have to be protected by subse-* quent treatments, such as lacquering or waxing, to provide aprotective coating thereover which must be renewed from time to time.

The surface is washed and then immersed in the dip of the present invention so as to provide the blackened coating.

One of the requirements of the solution for blackening a copper or copper alloy siu'face is that a minimum pH must be exceeded. This min- Another process involves the use of copper salt, v

such as copper sulphate, copper carbonate, or basic copper carbonate with ammonia in a bath. This process is used for applying blue black to hi-ass (copper zinc alloys) in the range of 65-35 to 80-20 copper zinc ratios by weight. In proper operation of this process, when copper carbonate is employed, for example, an excess of copper carbonate is required. Alloys containing higher copper contents than 80%, particularly the pure copper, are not blackened by this process to a deep black, but are colored a grayish black.

Still another method of blackening'copper and copper alloys involves the used sodium, 'potassium or ammonium persulphates, together with an alkali, such as sodium hydroxide or potassium hydroxide. This method, however, is very unsat isfactory because the persulphates are unstable and the life of the solution is relatively short, i. e.,

The present invention overcomes the diflicul ties of the prior process by providing a process and solution for direct oxidation of copper and copper alloy surfaces at low temperatures, less than 250 F., which process does not evolve irritating or harmful fumes, and the solution em-- ployed being stable over a period of days and This is accomplished by providing immersion of the surfaces in a hot aqueous bath of the coloring solution composed of sodium chlorite or potassium chlorite and either sodium hydroxide or potassium hydroxide.

In order to provide a uniform blackened surface by the process of the present invention, the surface may be cleaned and prepared by subjecting it to well known sulphuric acid-nitric acid bright dips, various concentrations of nitric acid and water, or etches containing chromic acid, chromates or dichromates with sulphuric acid.

coatings can be obtained in aqueous solutions having concentrations ranging between the following limits: sodium chl0rlte-5 g./l., sodium hydroxide-40 g./l. to sodium chlorite-saturated solution, sodium hydroxide-1000 g./l. The lower range of concentration requires approximately thirty minutes to blacken copper surfaces with the solution. operating at boiling point, whereas copper surfaces blacken in solutions of the high limits in approximately one minute when operated at boiling point.

These examples are given for copper surfaces. Usually somewhat higher temperatures and concentrations are employed for blackening copper alloys.

It has beeniound that coatings producedin solutions containing high concentrations of sodium hydroxide (above 200 g./l.) have a tan smut covering the black film underneath it. This smut can be easily wiped off exposing the black surfaces.

In normal commercial operation, however, the concentrations are maintained sumciently low so that'the smut does not appear. One proportion which has been very successful commercially includes one part by weight sodium chlorite to two parts by weight of sodium hydroxide, and this used at the rate of one to two pounds per gallon of water to form the bath which is most efficiently operated at from 200 F. to the boiling point of the solution.

In carrying out the process, the temperature required for the operation varies with the concentration of the component chemicals, the high concentrations permitting lower temperatures to be used. For example, with a concentration of sodium hydroxide of 10 g./l. and sodium chlorite at 10 g./l. a black color was obtained upon a copper surface at 175 F. in eighteen minutes, whereas with a concentration-of sodium hydroxide g./l. and sodium chlorite 150 g./l. a black was obtained upon a copper surface in fifteen minutes at 125 F.

By increasing the temperatures, the time required to produce the black surface may be effectively shortened. For example, the concentration of sodium hydroxide g./l. and sodium chlorite 10 g./l. can produce a black surface on copper in ten minutes if the temperature is raised to a boiling point of 216 F., and with the concentration of sodium hydroxide 150 g./l., sodium chlorite 150 g./l., black can be obtained in thirty seconds if the temperature of the bath is raised to the boiling point of the solution, namely. 250 F.

The black surfaces produced by the process are cupric oxide. These deep black surfaces are highly desirable in the manufacture of optical equipment, instruments, buttons and many other similar articles. Th appearances of these surfaces may be enhanced to present a greater depth of color by oiling, waxing or lacquering them.

The present invention is not limited to coating articles of solid copper or copper alloys, but may be used to coat surfaces of other metals which have been plated with copper or copper alloys when the coating thickness is greater than 0.00005" and the copper content of thealloy is greater than 60%.

It has been found that alkalies having lower pH in solution than sodium or potassium hydroxides, such as borax, trisodium phosphate, sodium metasilicate and sodium carbonate, are

ineffective for the present purpose when usedwith sodium chlorite. Also, it has been found that other oxidizing agents, such as potassium permanganate, sodium chlorate, sodium nitrate and sodium nitrite when mixed with caustic soda have proven ineffective to produce the black surface.

Variations and modifications may be made within the scope of this invention and portions of the improvements may be used without others.

I claim:

1. The process of producing a black surface on copper or copper alloy articles which comprises immersing the surface in a hot aqueous solution consisting essentially of an alkali selected from the group consisting of sodium hydroxide and potassium hydroxide and a chlorite selected from the group consisting of sodium chlorit and potassium chlorite.

2. The process of producing a black surface on copper or copper alloy surfaces which comprises immersing the surface in a hot aqueous solution of sodium chlorite and of an alkali selected from the group consisting of sodium hydroxide and potassium hydroxide, the concentration of the sodium hydroxide or potassium hydroxide and sodium chlorite being between about sodium chlorite-5 g./l., sodium hydroxide or potassium hydroxide-10 g./l. to sodium chlorite-saturated solution, sodium hydroxide or potassium hydroxide1000 g./l.

3. The process of producing a black surface on copper or copper alloy articles, which comprises immersin the surface in a hot aqueous solution of sodium hydroxid and sodium chlorite having a concentration between about sodium chlorite-5 g./l., sodium hydroxide-l0 g./l. to sodium chlorite-saturated solution, sodium hy- CIIOXidB-IOOO g./l.

WALTER R. MEYER. 

